Metal-gas cells, particularly nickel-hydrogen cells, are known in the art. These cells are contained in a sealed vessel or casing which contains hydrogen gas under high pressure. Each cell has at least one nickel-containing positive electrode which is spaced from a catalytic negative electrode. Electrodes are generally in the form of plates which are stacked together to form a plate stack. The plate stack includes gas access plates and separators which prevent short circuiting contact between the electrodes and which are saturated with a liquid electrolyte to provide desired cell perfor- mance.
The electrolyte is typically an alkaline medium such as an aqueous solution of alkali metal hydroxide, such as an approximately 30% potassium hydroxide solution. The negative catalytic electrode is a metal powder bonded within a plastic matrix. The metal powder is preferably one such as platinum or palladium black which will catalyze the oxidation or dissociation of hydrogen gas in an aqueous electrolyte. The plastic matrix is desirably tetrafluoroethylene such as "Teflon" brand material made by duPont. The active nickel-containing positive electrode is generally a nickel-oxy-hydroxide.
These metal-gas cells are, by preference, electrolyte starved cells. This refers to the quantity of electrolyte within the cell. Generally, the plate stack, including the electrodes and separators, will approach saturation with electrolyte and there may be a slight excess of electrolyte added. However, the majority of the interior of the cell casing which is not occupied by the plate stack is filled with gaseous hydrogen. This is required to provide sufficient hydrogen to react and to provide a gas-filled space to accommodate gases that are generated during the charging or discharging of the cell.
The pressure vessel or casing generally achieves superatomspheric pressure, for example, 20-50 atmospheres. Hydrogen in the vessel passes through an access plate to reach a catalytic negative electrode. The negative electrode causes molecular hydrogen to dissociate into atomic hydrogen which in turn reacts with free hydroxyl groups in the electrolyte to form water plus free electrons. The water and free electrons react with the nickel-oxy-hydroxide positive electrode to form nickel-hydroxide plus free hydroxyl ions.
During charging, opposite reactions occur so that the nickel-hydroxide forms nickel-oxy-hydroxide, water, and free electrons. The reformed active materials tend to be more amorphous than the active materials originally present on the positive electrode. Thus, during repeated charging and discharging cycles, the active faces of the electrodes may tend to increase in volume, or "grow". Since these cells are electrolyte starved, this causes a substantial problem. This growth of the active faces of the positive electrodes compresses the separators, forcing out electrolyte from the separators. The expanded plates are of increased porosity; they capture the electrolyte. The separators become denser and less conductive. Therefore, the plate stack is no longer electrolyte balanced and overall cell efficiency decreases. This eventually contributes to cell failure.
Nickel-hydrogen batteries are quickly becoming the preferred electrical storage system for earth-orbiting satellites. The reasons for this are the long life of the nickel-hydrogen cell, its wide operating range and most importantly, its high energy density. Since these are used in satellites, it is extremely important that the longevity of the cell is maximized. As such, it is critical that some means is provided to compensate for the growth of the plate stack, and preventing effective depletion of electrolyte by providing additional electrolyte to the plate stack progressively as needed.